refactor(stunreachability): input required and must be an URL (#630)

Here we're refactoring stunreachability to not provide internally a
default input and to take in input an URL rather than a string.

The related ooni/spec change is https://github.com/ooni/spec/pull/227.

This diff has been extracted from https://github.com/ooni/probe-cli/pull/539.

Because the original diff was large, I'm splitting it in a set of
more easily manageable diffs.

The reference issue is https://github.com/ooni/probe/issues/1814, which
is complex enough to require us to proceed incrementally.

This diff WILL need to be backported to release/3.11.

internal/engine/allexperiments.go

@@ -198,7 +198,7 @@ var experimentsByName = map[string]func(*Session) *ExperimentBuilder{
 				))
 			},
 			config:      &stunreachability.Config{},
-			inputPolicy: InputOptional,
+			inputPolicy: InputStrictlyRequired,
 		}
 	},